Modified diffusion layer for use in a fuel cell system

ABSTRACT

A fuel cell diffusion layer providing a preferential path by which liquid reactants or byproducts may be supplied to or removed from a direct oxidation fuel cell is described. The modified diffusion layer will be typically on the cathode side of the fuel cell and its use is to eliminate or minimize flooding of the cathode diffusion layer area, which is a performance limiting condition in direct methanol fuel cells. In accordance with one embodiment of the invention, the diffusion layer includes a substrate that is coated with a microporous layer. A pattern may be embossed into the diffusion layer, to create preferential flow paths by which water will travel and thereby be removed from the cathode catalyst area. This avoids cathode flooding and avoids build up of potentially destructive pressure by possible cathodic water accumulation. This also provides a means for collecting cathode water for redirection In accordance with another aspect of the invention, the preferential path is established by applying a thicker microporous layer to the carbon cloth or carbon paper and drying it in such a fashion so that when it dries, the surface of the microporous layer cracks to provide the pathways.

This application is a divisional of application Ser. No. 10/078,728,filed Feb. 19, 2002 now U.S. Pat. No. 6,890,680.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to direct oxidation fuel cells, andmore particularly, to diffusion layers for such fuel cells.

2. Background Information

Fuel cells are devices in which an electrochemical reaction is used togenerate electricity. A variety of materials may be suited for use as afuel depending upon the materials chosen for the components of the cell.Organic materials, such as methanol or natural gas, are attractivechoices for fuel due to the their high specific energy.

Fuel cell systems may be divided into “reformer-based” systems (i.e.,those in which the fuel is processed in some fashion to extract hydrogenfrom the fuel before it is introduced into the fuel cell system) or“direct oxidation” systems in which the fuel is fed directly into thecell without the need for separate internal or external processing. Mostcurrently available fuel cells are reformer-based fuel cell systems.However, because fuel processing is expensive and requires significantvolume, reformer based systems are presently limited to comparativelyhigh power applications.

Direct oxidation fuel cell systems may be better suited for a number ofapplications in smaller mobile devices (e.g., mobile phones, handheldand laptop computers), as well as in some larger applications.Typically, in direct oxidation fuel cells, a carbonaceous liquid fuel inan aqueous solution (typically aqueous methanol) is applied to the anodeface of a membrane electrode assembly (MEA). The MEA contains aprotonically-conductive but, electronically non-conductive membrane(PCM). Typically, a catalyst which enables direct oxidation of the fuelon the anode is disposed on one surface of the PCM (or is otherwisepresent in the anode chamber of the fuel cell). Protons (from hydrogenfound in the fuel and water molecules involved in the anodic reaction)are separated from the electrons. The protons migrate through the PCM,which is impermeable to the electrons. The electrons thus seek adifferent path to reunite with the protons and oxygen molecules involvedin the cathodic reaction and travel through a load, providing electricalpower.

One example of a direct oxidation fuel cell system is a direct methanolfuel cell system or DMFC system. In a DMFC system, methanol in anaqueous solution is used as fuel (the “fuel mixture”), and oxygen,preferably from ambient air, is used as the oxidizing agent. There aretwo fundamental reactions that occur in a DMFC which allow a DMFC systemto provide electricity to power consuming devices: the anodicdisassociation of the methanol and water fuel mixture into CO₂, protons,and electrons; and the cathodic combination of protons, electrons andoxygen into water. The overall reaction may be limited by the failure ofeither of these reactions to proceed to completion at an acceptable rate(more specifically, failure to oxidize the fuel mixture will limit thecathodic generation of water, and vice versa).

Typical DMFC systems include a fuel source, fluid and effluentmanagement systems, and a direct methanol fuel cell (“fuel cell”). Thefuel cell typically consists of a housing, and a membrane electrodeassembly (“MEA”) disposed within the housing. A typical MEA includes acentrally disposed protonically-conductive, electronicallynon-conductive membrane (“PCM”). One example of a commercially availablePCM is Nafion® a registered trademark of E.I. Dupont Nemours andCompany, a cation exchange membrane based on perflouorocarbon polymerswith side chain termini of perflourosulfonic acid groups, in a varietyof thicknesses and equivalent weight. The PCM is typically coated oneach face with an electrocatalyst such as platinum, orplatinum/ruthenium mixtures or alloy particles. On either face of thecatalyst coated PCM, the electrode assembly typically includes adiffusion layer.

A conventional diffusion layer serves to evenly distribute liquids andgases across the electrodes. In the case of the anode, the diffusionlayer is used to evenly distribute the fuel/water mixture to a maximumnumber of contact points on the surface of the anode so that thegreatest surface area of the anode is utilized for methanolelectro-oxidation. On the cathode side, the diffusion layer dispersesoxygen so that it is more evenly introduced to the cathode face of thePCM to promote the oxygen electro-reduction, which produces water. Inaddition, flow field plates are often placed on the surface of thediffusion layers, but are not usually in direct contact with the coatedPCM. The flow field plates function to provide mass transport ofreactants and byproducts of the electrochemical reactions, and the flowfield plates may also have a current collection functionality, in thatthe flow field plates act to collect and conduct electrons to the load.

A typical diffusion layer may be fabricated of carbon paper or a carboncloth, typically with a microporous coating made of a mixture of carbonpowder and polytetrafluoroethylene (“PTFE), sold commercially asTEFLON®, a trademark of the E.I. DuPont de Nemours and Company. The PTFEcomponent has a function of wet proofing in the case of a gas-suppliedelectrode, but as the cell reaction proceeds, the carbon paper or carboncloth can become saturated with liquid water. This can be caused bycontinuous water build-up in the cathode chamber of the fuel cell. Thecathode can become “flooded”, in which case the cathode half of thereaction can be compromised or even prevented. This results in theoverall performance of the cell being compromised, or prevented.

Typically, the risk of cathode flooding is mitigated by active air flowto remove water from the cathode layer. This, however, increases thecost and complexity of the fuel cell system, thus adding to the expenseof manufacture, as well as introducing the possibility of parasiticlosses. In addition, it also adds volume to a system that must meetdemanding form factors.

It is also noted that when water builds up in the cathode, it not onlycan cause flooding, which reduces the effectiveness of the half reactionon the cathode side, but it also results in pressure on the cathode faceof the PCM that can weaken or compromise the bond between the membrane(PCM) and the catalytic coating, or the bond between the diffusion layerand the catalytic coating. Cell performance can be reduced over the longrun because these stresses can ultimately cause separation of key fuelcell components, preventing the effective operation thereof.

There remains a need therefore for a diffusion layer that providesoptimal gas diffusion properties, and resists flooding of the cathodeportion of the fuel cell. In addition, in direct methanol fuel cells(DMFCs), the removal and collection of liquid water from the cathode bymeans of such a modified cathode diffusion layer is of high significancein maintaining overall water balance in the fuel cell system. Effectivecollection of liquid water at the cathode may be prerequisite tocarrying just neat (pure) methanol, rather than methanol/water mixtures,as fuel supply to the DMFC.

It is thus an object of the invention to provide a diffusion layer thatreduces the risk of cathode flooding, and liquid water-causeddeterioration on the cathode side of the fuel cell and/or a cathodediffusion layer that allows liquid water to be collected for use in thedirect methanol fuel cell system.

SUMMARY OF THE INVENTION

The present invention is a modified diffusion layer for use on thecathode face of a protonically conductive membrane of a DMFC, which iscomprised of a diffusion material that has preferential flow pathsincorporated therein which redirect and remove liquid across thediffusion layer and cause the liquid to preferentially flow, in apredetermined manner, usually away from the PCM. The inventive diffusionlayer can thus provide a preferential path by which liquid reactants orbyproducts may be removed. By providing a means by which fluids presenton the cathode face of the fuel cell are removed without the use ofpumps or other power consuming devices, the overall efficiency of thefuel cell is enhanced, and the operation of the cell is improved.

In accordance with the invention, the diffusion layer includes asubstrate formed substantially of carbon, and is typically fabricatedfrom carbon cloth or carbon paper. The substrate is coated with ahydrophobic microporous layer of TEFLON® and a high surface area ofcarbon particles on at least one side. An indentation pattern is formedto create channels, in accordance with the invention, into themicroporous layer. The patterned channels provide paths by which fluidswill preferentially travel in the fuel cell. In the case of the cathode,the preferential paths, or channels, direct the water to either acollection point away from the PCM where it may be purged to the ambientenvironment, or it may be recirculated to return it to the anode side ofthe cell.

In accordance with the method of the present invention, the modifieddiffusion layer can be fabricated by forming a substrate substantiallyof carbon, such as carbon paper or carbon cloth. The substrate is thentreated by coating it with a microporous layer comprised ofTEFLON®-coated high surface area carbon particles. This microporouslayer is then embossed, using indentation techniques to form therein apattern, thus, producing preferential flow paths, to direct the water,or other fluids, away from the membrane, and to remove them from theactive area of the cell, as desired.

In accordance with yet a further aspect of the invention, thepreferential path is established by applying a thicker wet mixturecontaining TEFLON ® and high surface area carbon particles than istypically applied, to the carbon cloth or carbon paper. After drying andthen sintering at the glass transition temperature of TEFLON®, the faceof the porous coating layer cracks, forming a mud-cracked pattern on theresulting microporous layer. These mud cracking patterns extend to theedges of the diffusion layer, providing a pathway for water to betransported away from the active electrode area.

In addition, materials may be selectively chosen for the microporouslayer and other components in the cell, to encourage the flow of water(or other liquid byproducts and reactants) in certain predetermineddirections within the cell, to further enhance performance of the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a block diagram of a direct oxidation fuel cell system withwhich the diffusion layer of the present invention may be employed;

FIG. 2 is an isometric schematic drawing of a membrane electrodeassembly with which the present invention may be employed;

FIG. 3A is an isometric view of a membrane electrode assembly thatincludes the diffusion layer of the present invention and illustratesflow channels in the diffusion layer;

FIG. 3B is a top plan view of an embodiment of the diffusion layer ofthe present invention in which the flow channel is of a spiral shape;

FIG. 3C is a top plan view of an embodiment of the diffusion layer ofthe present invention in which the flow channel is of a lattice pattern;

FIG. 4A is a cross section of a membrane electrode assembly whichemploys a diffusion layer constructed in accordance with another aspectof the invention in which cracks are formed in the microporous layer;and

FIG. 4B is a top plan view of the embodiment of FIG. 4A.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

For a better understanding of the invention, the components of a directoxidation fuel cell system, a direct oxidation fuel cell and the basicoperation of a fuel cell system, will be briefly described. Oneembodiment of a direct oxidation fuel system 2 is illustrated in FIG. 1,though the invention set forth herein may be used in a number of othersystem architectures. The fuel cell system 2 includes a direct oxidationfuel cell, which may be a direct methanol fuel cell 3 (“DMFC”), forexample. For purposes of illustration we herein describe an illustrativeembodiment of the invention with DMFC 3, with the fuel substance beingmethanol or an aqueous methanol solution. It should be understood,however, that it is within the scope of the present invention that otherfuels may be used in an appropriate fuel cell. Thus, as used herein, theword “fuel” shall include methanol, ethanol, propane, butane orcombinations thereof and aqueous solutions thereof, and otherhydrocarbon fuels amenable to use in direct oxidation fuel cell systems.

The system 2, including the DMFC 3, has a fuel delivery system todeliver fuel from fuel source 4. An internal reservoir 4 a may, but neednot be, utilized in conjunction with the fuel source. Alternatively, arefillable internal reservoir may be used to store fuel. The DMFC 3includes a housing 5 that encloses a membrane electrode assembly 6(MEA). MEA 6 incorporates protonically conductive, electronicallynon-conductive membrane (PCM) 7, MEA 6 also incorporates an anodediffusion layer 8 and cathode diffusion layer 10, each of which may becoated with a catalyst, including but not limited to platinum, a blendof platinum and ruthenium, or other alloy with high surface areaparticles, which may be supported or unsupported by carbon particles.The portion of DMFC 3 defined by the housing 5 and the anode face of thePCM is referred to herein as the anode chamber 18. The portion of DMFC 3defined by the housing 5 and the cathode face of the PCM is referred toherein as the cathode chamber 20.

As will be understood by those skilled in the art, a carbonaceous fuelin an aqueous solution (typically an aqueous methanol solution) passesfrom a fuel source 4, through the anode flow field plate (if any) afterwhich it enters the anode diffusion layer 8 where it is dispersed andpresented to the anode aspect of the PCM 7, in a substantially uniformfashion. Similarly, an oxidizing agent (or oxidant), preferably ambientair, is made available to the PCM 7, via the cathode diffusion layer 10,the details of which are described herein after in accordance with thepresent invention. Those skilled in the art will recognize that flowfield plates (not shown) may be placed in contact with each aspect ofthe diffusion layers 8,10 that are not in contact with the PCM 7.

Catalysts on the PCM 7 (or are otherwise present in each of the anodeand cathode chambers, 18 and 20 respectively) enable the oxidation ofthe carbonaceous fuel and water mixture on the anode face 9 of the PCM 7forming carbon dioxide as the byproduct of the anodic reaction, andreleasing protons and electrons from the hydrogen atoms in the fuel andwater mixture. Upon the closing of an external circuit, the protons passthrough the PCM 7, which is impermeable to the electrons. The electronsseek a different path to reunite with the protons and travel through aload and, thus, provide the electrical power from the fuel cell 3. Theelectrochemical reactions are as follows:Anode: CH₃OH+H₂O═CO₂+6H⁺+6e ⁻  Equation 1Cathode: 6H⁺+6e ⁻+3/2O₂═3H₂O  Equation 2Net Process: CH₃OH+3/2O₂═CO₂+2H₂O  Equation 3

As stated before, the second half of the reaction occurs in the cathodeand it is described in the above Equation 2. More specifically, water isproduced at the cathode face of the PCM. Under some operatingconditions, so much water is created or passes through the PCM, that thecatalyst diffusion layer 10 and/or the cathode catalyst layer can becomeflooded causing the DMFC to cease functioning.

Referring now to FIG. 2, the diffusion layer of the present invention,which provides a solution to this problem, will be described in detail.A protonically-conductive membrane PCM 7 has a cathode face 11, which iscoated with a catalyst layer 30. A diffusion layer 10 is placedcontiguous to the catalyst layer 30. The diffusion layer 10 isfabricated as a substrate 44 formed substantially of carbon, such ascarbon cloth or carbon paper. A hydrophobic microporous layer comprisedof TEFLON®-coated high surface area carbon particles is typicallyapplied to the substrate 44, forming a microporous backing layer 48. Themicroporous backing layer 48 is in intimate contact with the catalystcoated membrane 7, in order to minimize resistance to the flow ofelectrons across the fuel cell.

In accordance with one aspect of the present invention, the diffusionlayer is embossed with a pattern, as illustrated with greater detailwith reference to FIGS. 3A–3C. More specifically, the cathode diffusionlayer 10 has embossed therein flow channels, which create a preferentialflow path upon which water (or a liquid containing water, but which mayalso include a fuel solution) will travel away from the cathode face 11of the membrane 7. Potential paths include but are not limited to thoseshown in the isometric view of FIG. 3A, the flow channels 50 and 52, forexample, allow the water to flow away from the center of the membraneand consequently the fluids may be collected and recirculated, oreliminated from the system. As shown in FIG. 3B, the flow channel can bein a spiral pattern 22. As shown in FIG. 3C, a top plan view, the flowchannels can be of a lattice layout having substantially linearportions, such as flow channel portions 22 and 24. These embossmentswill create a preferential liquid flow path to direct fluids as desiredin the cell, and in one exemplary embodiment, will direct liquid wateraway the cathode diffusion layer to prevent and resist flooding of thecathode and separation of the cathode components of the fuel cell. Itshould be understood that many other geometric patterns may be formed asflow channels in the diffusion layer, while remaining within the scopeof the present invention.

While not limiting to the invention, the flow channels may also minimizebuildup of pressure by allowing liquid that accumulates at the interfacebetween the cathode catalyst and the cathode microporous backing layer48, (also referred to herein as a microporous layer), an outlet, so thathydrostatic pressure that could be created by such liquid does not buildup. Instead, in the presence of the embossed flow channels the liquid isremoved by the small hydrostatic pressure build up.

In addition, the flow channels may also provide an outlet to release thewater built up from the cathodic reaction and the accompanying watertransport through the membrane. Water generated in the active electrodearea, where the hydrophobic microporous layer is in direct and intimatecontact with the catalyst layer, will be pushed away from the activeelectrode area and into the flow channels by the hydyrostatic pressuregenerated by the capillary force of the hydrophobic microporus layer. Itthus opens the one-way transportation of oxygen from the air to thecathode catalyst layer without the egress flow of water encountered witha non-improved backing. Without water accumulation between the cathodecatalyst layer and microporous layer or between the catalyst layer andmembrane by using the improved cathode backing, the risk of cellcomponent delamination may be eliminated. The hydrostatic pressure thatdrives water into the channels formed in the microporous layer may alsobe used to collect and direct the water from the cathodic reaction fromthe cathode compartment to the anode compartment passively, thusminimizing excessive water loss from the cathode. As a result, a higherpower pack energy density can be achieved by carrying more methanol andless water as the fuel mixture.

The flow channels direct the water either to a collection point such ascollection point 60 (FIG. 3C) or it may be eliminated into the ambientenvironment, or it may be returned to the anode side of the cell or ofanother cell as shown in FIG. 1. Those skilled in the art will recognizethat the invention may also be used where a stack or other assemblycontaining more than one fuel cell is connected.

The pattern on the diffusion layer can be formed not only by embossing,but by other mechanical means that are appropriate in the particularapplication in which the invention is being employed such as milling orcasting the microporous layer to form the flow channel pattern into thecomponent in accordance with the present invention. The milled or castlayer would then be bonded or otherwise attached to the substrate of thediffusion layer using methods known to those skilled in the art.

Another aspect of the invention will be described with reference toFIGS. 4A and 4B. More specifically, in accordance with yet a furtheraspect of the invention, the preferential path is established byapplying a thicker wet mixture containing TEFLON®and high surface areacarbon particles, to the carbon cloth or carbon paper than is typicallyapplied. After drying, then sintering at the glass transitiontemperature of the TEFLON®, the face of the porous coating layer cracks,forming a mud-cracked pattern 400 on the resulting microporous layer asshown in FIG. 4B. These mud cracking patterns can extend to the edges ofthe diffusion layer 402, 404, providing a pathway for water to betransported away from the active electrode area, to a point that is awayfrom the active area of the PCM. The cracks would not extend into themembrane, but instead would extend from the surface of the microporousbacking layer, partially towards the surface of the substrate.

A commercially available diffusion layer, such as ELAT®, may also bemodified by applying an additional layer of TEFLON® coating (or anadditional layer of the material used to fabricate the microporouslayer) to one side of said diffusion layer, then causing such layer tocrack, using the same methods as set forth above. Whether the diffusionlayer is fabricated from raw materials or is fabricated by modifying anexisting diffusion layer, the cracks will, at most, penetrate thediffusion layer only to the depth of the substrate, as noted. Themicroporous layer opposite the face of the diffusion layer into whichthe preferential flow channels are established remains entirely intact,thus providing a diffusion layer with a preferential path withoutcompromising the structural or conductive integrity of said diffusionlayer. Alternate methods of cracking the microporous layer are withinthe scope of the invention. It should be understood that this embodimentmay also be easier to manufacture, while still providing the benefits ofremoval of water from the active cathode area and protecting the PCM.

Performance may also be enhanced by adding hydrophobicity orhydrophilicity character to the appropriate materials in the cell whichwould also encourage and facilitate water removal from the cell orcathode and to encourage other byproducts to travel in such a directionso as to allow the energy generating reactions of the fuel cell systemto proceed more efficiently to completion.

The method of the present invention includes the steps of making animproved diffusion layer by fabricating a substrate substantially ofcarbon, such as fabricating the substrate out of carbon paper or carboncloth. The substrate is then treated by coating it with an electricallyconductive layer, which is substantially hydrophobic, but permeable togases, thus forming a microporous layer over the substrate. Themicroporous layer may be a composite of TEFLON® and high surface areacarbon particles. This microporous layer is then embossed, by highpressure indentation techniques to form therein a pattern, thus,producing a preferential flow path, to direct water, or other liquidsfluids, away from the membrane, and remove them from the active area ofthe cell, as desired. Instead of embossing, other mechanical techniquesmay be used such as milling and casting to form a layer having flowchannels therein.

The method of the present invention for fabricating a liquid evolving,which may also be referred to herein as a liquid removing diffusionlayer, may be further understood with reference to the accompanyingexample, which is illustrative only and not limiting to the invention.

EXAMPLE

A diffusion layer was fabricated by employing a substrate formed from asheet of ELAT® diffusion backing, commercially available from the E-Tekdivision of De Nora N.A., 39 Veronica Avenue, Somerset, N.J. 08873,measuring about 3.162 cm by 3.162 cm. This diffusion layer was comprisedof a carbon cloth substrate with a microporous layer comprised ofTEFLON®-bonded high surface area carbon particles. A lattice likepattern was embossed into the diffusion layer using-pressure of 10,000pounds per square inch, the pattern extending to the edges of thediffusion layer. The embossed diffusion layer was then placed inintimate contact with a catalyst coated membrane for use in the membraneelectrode assembly of a direct methanol fuel cell.

The microporous layer tends to form a hydrophobic barrier adjacent thecatalyst coated PCM, however, water can still build up near the cathodeface of the fuel cell. The embossment in the cathode diffusion layercauses such water to travel along the preferential flow paths and beremoved from the cathode, thus reducing the risk of cathode floodingwhich could limit the flow of oxygen to the cathode face, and limitingcell performance. It should be understood by those skilled in the artthat the diffusion layer of the present invention allows gaseousreactants to reach the membrane while removing liquid byproducts fromthe membrane. The diffusion layer, on the cathode side, allows oxygen todiffuse through the microporous layer to the cathode catalyst layer,while the water produced in the cathode half reaction as well as watercrossing through the membrane is directed away from the membrane. Inaddition, the diffusion layer of the present invention protects thecathode side of the membrane by reducing hydrostatic pressure that canotherwise cause delaminating problems.

It should be understood that the diffusion layer of the presentinvention assists in prevention of cathode flooding without theapplication of external force or energy. More specifically, the presentinvention provides a method of water removal that does not requireactive drying of the cathode layer.

Thus, it should be understood that the diffusion layer of the presentinvention provides many advantages for use with a low temperature directoxidation fuel cell, where water tends to build up at the cathodecatalyst and is required to be collected and redirected to maintainwater balance. While we have described the diffusion layer with respectto the cathode, there may also be instances in which a patterneddiffusion layer that includes preferential flow paths may beadvantageously employed in the anode chamber of the fuel cell, whileremaining within the scope of the present invention.

The foregoing description has been directed to specific embodiments ofthe invention. It will be apparent however that other variations andmodifications may be made to the described embodiments with theattainment of some or all of the advantages of such. Therefore, it isthe object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

1. A method of fabricating a liquid removing diffusion layer for aprotonically conductive membrane used in a direct oxidation fuel cell,the method including the steps of: (A) providing a substratesubstantially of carbon; (B) treating the substrate by coating it with amicroporous layer of material that is electrically conductive,hydrophobic and permeable to gases, including providing a coating of athickness that, when dried, the layer tends to fracture and results incracks, said cracks thus forming channels, producing preferential flowpaths to direct liquids in predetermined directions in said fuel cell.2. The method as defined in claim 1 wherein said liquid is water andsaid channels form a flow path that directs the water away from themembrane to remove water from an active area of the fuel cell.
 3. Themethod as defined in claim 1 wherein said step of providing saidsubstrate includes providing a substrate that is a sheet of carbonpaper.
 4. The method as defined in claim 3 wherein said step ofproviding said substrate includes providing a substrate that is aplurality of sheets of carbon paper.
 5. The method as defined in claim 1wherein said step of providing said substrate includes providing asubstrate of carbon cloth.
 6. The method as defined in claim 1 whereinsaid microporous layer is comprised substantially ofpolytetrafluoroethylene and a dispersion of high surface area carbonparticles.
 7. The method as defined in claim 1 including the furtherstep of selecting as a material for said microporous layer, a materialthat encourages the flow of water-containing liquids in certainpredetermined directions within the cell.
 8. A method of fabricating aliquid removing diffusion layer for a protonically conductive membraneused in a direct oxidation fuel cell, the method including: (A)providing a substrate substantially of carbon; (B) treating thesubstrate by coating it with a microporous layer of material that iselectrically conductive, hydrophobic and permeable to gases by applyinga wet mixture containing polytetrafluoroethylene and high surface areacarbon particles to said substrate as said microporous layer; (C) dryingsaid microporous layer; and (D) allowing said layer to form cracks assaid channels to produce said preferential flow paths.
 9. The method asdefined in claim 8 wherein said drying step includes sintering saidmicroporous layer at a glass transition temperature ofpolytetrafluoroethylene.
 10. The method as defined in claim 8 whereinsaid step of providing said substrate includes providing a gas diffusionlayer being a microporous layer on a woven web and said treating stepincludes adding an additional layer of a substance containingpolytetrafluoroethylene.